This application is based on Japanese Patent Application No. 2001-211634 filed Jul. 12, 2001, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a timing synchronization device of a pulsed laser and an optical synthesizer. More specifically, the invention relates to a timing synchronization device of a femtosecond pulsed laser that is widely used as an ultrashort pulse light source in optical communications, medical care, and other science and technology fields as well as to an optical synthesizer using it.
2. Description of the Related Art
The following two methods are typical of the conventional sync control techniques of femtosecond pulsed lasers.
In the first method, sync control is performed by detecting pulse intervals of independent ultrashort pulse laser beams having different wavelengths with electric circuits such as counters, determining their deviation value, and feeding back the deviation value to a resonator-mirror-driving piezoelectric transducer that is provided in one of the pulsed lasers.
In this method, the timing jitter is determined by the resolutions of the electric circuits and the piezoelectric transducer, and is of the order of picoseconds on the product level (reference 1: General catalogue xe2x80x9800-xe2x80x9901 of Spectraphysics Corp., p. 18, xe2x80x9clok-to-clockxe2x80x9d) and of the order of 10-20 femtoseconds on the research level (reference 2: R. K. Shelton et al., international conference CLEO 2001, CTuG2).
In the second method, two resonators are produced for one laser gain medium. It has been reported that two timing-synchronized pulse laser beams having difference wavelengths can be output from a Ti:sapphire laser having Ti:sapphire as a laser gain medium (reference 3: S. J. White et al., international conference CLEO 2000, CThB5; reference 4: J. M. Evans et al., Opt. Lett., 18, p. 1,074, 1993; reference 5: A. Leitenstorfer et al., Opt. Lett., 20, p. 916, 1995; reference 6: M. R. X. de Barros et al., Opt. Lett., 18, p. 631, 1993; and reference 7: D. R. Dykaar et al., Opt. Lett., 18, p. 634, 1993).
It is considered that in this method two pulse laser beams cause four-wave mixing in one laser gain medium to establish timing synchronization.
However, the first method has a problem that the influence of timing jitter is too large to perform correct sync control on a femtosecond pulsed laser.
The second method has a problem that the wavelengths of pulse laser beams produced by a two-wavelength timing-synchronized laser having Ti:sapphire as a laser gain medium are restricted to only two close wavelengths that are determined by the characteristics of the laser gain medium because only one laser gain medium is used. There is another problem that the two pulse laser beams are produced by using Ti:sapphire as the laser gain medium and hence are rendered unstable in a state of good pulse overlap because of occurrence of gain competition. Because of this phenomenon, in two-wavelength timing-synchronized lasers having Ti:sapphire as a laser gain medium, it is difficult to synchronize two pulse laser beams having different wavelengths with each other in a stable manner.
The present invention has been made in view of the above problems in the art, and an object of the invention is therefore to provide a timing synchronization device of a pulsed laser capable of timing-synchronizing two ultrashort pulse laser beams whose wavelengths are separated from each other to some extent as well as an optical synthesizer using it.
The invention employs the following configurations to attain the above object.
The invention provides a first timing synchronization device of a pulsed laser, comprising a first laser gain medium having a first gain region; a second laser gain medium having a second gain region that is not included in the first gain region; and first and second resonators provided with the first and second laser gain media, respectively, and arranged so that two laser pulses occurring in the first and second resonators, respectively, overlap with each other in one of the first and second laser gain media, wherein the second laser gain medium has an oscillation wavelength at which a crystal of the first laser gain medium cannot cause oscillation.
A second timing synchronization device according to the invention is such that, in the first timing synchronization device, group velocity dispersion of each of the first and second resonators as a whole is set minus, and a leading laser pulse changes a refractive index characteristic of the one laser gain medium to a red-shift-inducing characteristic or a blue-shift-inducing characteristic and a lagging laser pulse is caused to pass through the one laser gain medium, whereby the leading laser pulse and the lagging laser pulse are synchronized with each other.
A third timing synchronization device according to the invention is such that, in the first timing synchronization device, a first pulse laser beam produced by using the first laser gain medium and a second pulse laser beam produced by using the second laser gain medium are synchronized with each other in such a manner that an frequency spacing among adjacent longitudinal modes of the first pulse laser beam and that of the second pulse laser beam are made identical through four-wave mixing.
A fourth timing synchronization device according to the invention is such that, in the first timing synchronization device, the first gain medium is Ti:sapphire and the second gain medium is Cr:forsterite.
A fifth timing synchronization device according to the invention is such that, in the first timing synchronization device, the first laser gain medium is Ti:sapphire and the second laser gain medium is Cr4+-doped YAG.
The invention provides a first optical synthesizer comprising any of the first to fifth timing synchronization devices; and a nonlinear crystal, wherein first and second pulse laser beams having different first and second optical frequencies xcfx891 and xcfx892, respectively, that are output from the timing synchronization device are input to the nonlinear crystal, and wherein the optical synthesizer outputs, through the nonlinear crystal, a sum-frequency beam having a sum frequency xcfx893 (=xcfx891+xcfx892) of the first and second frequencies and a differential frequency beam having a differential frequency xcfx894(=xcfx891xe2x88x92xcfx892) of the first and second frequencies, outputs, through the nonlinear crystal, a second-harmonic pulse of the first beam having a frequency 2xc2x7xcfx891 that is two times higher than the optical first frequency, and outputs, through the nonlinear crystal, a second harmonic pulse of the second beam having a frequency 2xc2x7xcfx892 that is two times higher than the second optical frequency, and outputs the first and second pulse laser beams as they are.
A second optical synthesizer according to the invention is such that, in the first optical synthesizer, the first and second frequencies correspond to wavelengths of 850 nm and 1,275 nm, respectively, and the sum frequency corresponds to a wavelength of 510 nm.
A third optical synthesizer according to the invention is such that, in the first optical synthesizer, the optical synthesizer combines arbitrarily selected ones of the first and second pulse laser beams, the sum-frequency beam, the differential frequency beam, and the first and second second harmonic beams, and outputs a resulting beam.
In the invention, the second laser gain medium has an oscillation wavelength at which the first laser gain medium crystal cannot cause oscillation, whereby pulse laser beams having much different oscillation wavelengths can be synchronized with each other.
In the timing synchronization device of a two-wavelength pulsed laser according to the invention, two pulse beams having different wavelengths are generated by using two laser gain media and synchronized with each other in a passive manner. As a result, two pulse beams having different wavelengths having a very low degree of timing jitter.
Since the laser gain medium for timing synchronization is made of a material that is transparent to one pulse beam, no gain competition occurs and more stable timing synchronization can be attained.
Further, the optical synthesizer according to the invention can produce an optical field having an arbitrary waveform and can easily generate RGB signals for color image display.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.